1. Field of the Invention
The invention relates to a method for correcting oscillation-induced imaging errors in an objective, in particular in a projection objective for microlithography for fabricating semiconductor elements, the objective having at least one first objective part with a first optical axis and a second objective part whose optical axis deviates from the first optical axis, beam deflection taking place between the two objective parts via at least one optical beam deflection element.
The invention also relates to an apparatus for correcting oscillation-induced imaging errors in an objective.
Furthermore, the invention relates to a projection objective for microlithography for fabricating semiconductor elements, having at least one first objective part with a first optical axis and having a second objective part whose optical axis deviates from the first optical axis, an optical beam deflection element being arranged between the at least one first objective part and the second objective part.
2. Description of the Related Art
For spatial reasons, objectives, such as e.g. projection objectives for microlithography for fabricating semiconductor elements, are often designed in such a way that a plurality of optical axes are present between an object plane and an image plane. Thus, e.g. so-called catadioptric objectives are known, an objective part having a horizontal optical axis or an optical axis that is inclined slightly downward with respect to the horizontal. In this case, beam deflection takes place on the input side from a first objective part with a vertical optical axis via a prism to a horizontal objective housing. In the horizontal objective part, beam reflection takes place after passage through a group of lenses via a concave mirror back to the prism, from where the beam passes once again with a vertical optical axis to the image plane.
In the case of a similar type of objective, a first vertical optical axis is followed by an optical axis which runs in the horizontal direction and likewise runs in a horizontal objective part. Here, too, the beam is reflected at a concave mirror. In this case, a beam splitter element, e.g. a beam splitter cube, is provided as beam deflection device between the vertical and horizontal axes, the beam reflected at the concave mirror, after passing through the beam splitter element, being deflected at a deflection mirror to a third objective part with a second vertical axis. In this case, the image plane is situated below the third objective part, a wafer being situated in said image plane in the case of a projection objective for microlithography. Arranged in the object plane, which is situated above the first objective part in the case of both types of objective, is a reticle whose structure is imaged on the wafer on a correspondingly reduced scale.
In the case of both types of objective, the second objective part with its objective housing must be made very stiff since dynamic displacements and tiltings of the concave mirror mounted in the horizontal part of the objective housing greatly affect image errors of the objective. What is disadvantageous in this case is, in particular, that in the case of the high imaging accuracies demanded in the case of projection objectives for microlithography, despite a solid mechanical construction, occasionally the imaging qualities demanded cannot be achieved or can only be achieved with a very high outlay.